The present invention relates to an optical information recording medium and, more particularly, to a phase change type optical information recording medium in which a structure is reversibly changed between the crystalline and amorphous states, and an optical nature is changed in accordance with a thermal hysteresis such as temperature rise and cooling by being irradiated with a laser beam.
Since the recording scheme for an optical information recording medium employing a laser beam, and particularly the recording scheme for a disk-shaped optical information recording medium (to be referred to as an optical disk hereinafter) enable large-capacity recording and high-speed access in the non-contact manner, they have been put into practical use in the form of large-capacity memories. Optical disks are classified into ROM optical disks known as compact disks (CD) and laser disks, write-once optical disks on which the user can record information, and re-writable optical disks on and from which the user can repeatedly record and erase information. The write-once and re-writable optical disks are used as external memories for computers, or document and image files.
The re-writable optical disks include a phase change type optical disk that utilizes the phase change of a recording film and a magneto-optical disk that utilizes changes in magnetization direction of a perpendicular magnetized film. Of these disks, the phase change type optical disk is expected to be popular as a re-writable optical disk because it does not require an external magnetic field and can easily perform an overwrite operation.
Conventionally, the phase change type optical disk capable of a write access using a recording film that undergoes a phase change between the crystalline phase and the amorphous phase in accordance with laser beam irradiation has been used. A laser beam spot having a high power in accordance with information to be recorded on the recording film is irradiated on the phase change type optical disk to locally increase the temperature of the recording film. Thus, a phase change between the crystalline phase and the amorphous phase is caused to perform recording. A change in optical constant accompanying this phase change is read by a low-power laser beam as a difference in intensity of the reflected light, thereby reproducing the information.
For example, when a phase change type optical disk using a recording film having a comparatively long crystallization time is employed, the optical disk is rotated, a laser beam is irradiated on the recording film formed on the optical disk to heat a portion of the recording film to a temperature equal to or higher than the melting temperature. After the laser beam passes, the heated portion is rapidly cooled to set this portion in the amorphous state, thereby performing recording.
For erasing, the recording film is held within a crystallization enable temperature which is greater than or equal to a crystallization temperature and less than the melting temperature for a sufficiently long period of time required for promoting crystallization, so that the recording film is crystallized. As a method for this, a method of irradiating a laser beam having an elliptic spot in the laser beam traveling direction of the disk is known. To perform a pseudo-overwrite operation with two beams in which new information is recorded while erasing already recorded data, an erasing laser beam having an elliptic spot is irradiated to the recording film preceding a recording laser beam having a circular spot.
When a disk using an information recording film capable of high-speed crystallization is employed, a single laser beam focused to have a circular spot is used. According to a conventional method, a phase change between the crystalline and amorphous states is caused by changing the power of the laser beam between two levels. More specifically, when a laser beam having a high power that can heat the recording film to a temperature equal to the melting temperature or more is irradiated on the recording film, most of the recording film is set in the amorphous state upon being cooled. Meanwhile, when a laser beam having a low power that heats the recording film to a temperature which is greater than or equal to the crystallization temperature and less than the melting temperature is irradiated on the recording film, most of the recording film is set in the crystalline state.
To form the recording film of a phase change type optical disk, GeSbTe, InSbTe, InSe, InTe, AsTeGe, TeO.sub.x -GeSn, TeSeSn, SbSeBi, or BiSeGe belonging to a chalcogenide-based material is employed. Any material is deposited in accordance with resistance heating vacuum deposition, electron beam vacuum deposition, sputtering, or the like. The recording film immediately after deposition is set in a kind of amorphous state. The entire recording film is set in the crystalline state by initialization so that an amorphous recorded portion is formed upon recording information on the recording film. Recording is achieved by forming an amorphous portion in the recording film which is in the crystalline state.
As a conventional technique for performing high-density recording in such a phase change type optical disk, a mark edge recording scheme of recording information at a recording mark edge has been proposed. In recording information at a mark edge, the edge position must be accurate. Fluctuations in edge position greatly limit high-density recording. The main cause of this edge fluctuation is an absorption difference between the amorphous and crystalline states. To suppress this edge fluctuation, it is importance to set the absorption of a portion in the crystalline state higher than that of an amorphous portion.
A well-known optical disk using a metal reflective film has an arrangement in which a reflectance difference between portions in the crystalline and amorphous states is set large. In general, since the reflectance of the crystalline portion is set higher than that of the amorphous portion, the absorption of the amorphous portion is inevitably increased to increase the mark edge fluctuation in overwrite recording, as described above.
Absorption control will be described below in detail. A well-known phase change type optical disk has a four-layered reflective film arrangement in which a first protection film 2, a recording film 3, a second protection film 4, a metal reflective film 5, and an ultraviolet-curing resin 6 are sequentially stacked on a transparent substrate 1, as shown in a sectional view of FIG. 3.
In general, the above-described arrangement is employed in which the reflectance of an amorphous portion is set lower than that of a crystalline portion, thereby assuring a reproduced signal by setting the reflectance of the crystalline portion higher than that of the amorphous portion. In this case, a change in optical constant accompanying a phase change in recording film can be effectively converted into a change in reflectance, thereby assuring a good reproduced signal. To the contrary, most of light is reflected by the metal reflective film 5. For this reason, if a large reflectance difference accompanying the change in the optical constant of the recording film 3 is to be assured, the absorption of the crystalline portion of the recording film 3 inevitably becomes lower than that of the amorphous portion.
If the absorption of the crystalline portion is decreased in this manner, the temperature rise state of the recording film varies depending on the crystalline or amorphous state of the recording film in an overwrite operation because the thermal conductivity in the crystalline state is higher than that in the amorphous state, and the latent heat energy required for melting is also larger. Therefore, an overwrite signal is modulated due to a signal component before an overwrite operation. This is one factor of the mark edge fluctuation which limits a decrease in overwrite jitter.
To eliminate this mark edge fluctuation, three methods have been proposed: 1 a method using a very thin metal reflective film; 2 a method using a transparent high-refractive-index dielectric material as a reflective film; and 3 a method in which information is reproduced using an optical phase difference, and the absorption is set with a certain degree of freedom.
In the first method, gold (Au) is used as a very thin metal reflective film, and the optical absorption of the recording film in the crystalline state is set equal to that in the amorphous state to obtain a similar temperature rise profile in view of time and space (Japanese Patent Laid-Open No. 1-149238). In this conventional method, however, the metal reflective film is as thin as, e.g., 20 nm, so that the repeated overwrite characteristics are poor.
The second method using a transparent high-refractive-index dielectric material as a reflective film uses a transparent high-refractive-index dielectric material made of silicon (Si) as a reflective film (Japanese Patent Laid-Open No. 4-102243). That is, according to this method, an optical information recording medium has an arrangement in which a first protection film 2, a recording film 3, a second protection film 4, and a high-refractive-index dielectric reflective film 7 made of Si, and an ultraviolet-curing resin 6 are sequentially stacked on a substrate 1.
FIG. 5 shows the dependence of the reflectance and transmittance of a conventional optical information recording medium on the film thickness of the first protection film 2 as a lower protection film. As shown in FIG. 5, with a change in the film thickness of the first protection film 2, an absorption I of a portion in the crystalline state, an absorption II of a portion in the amorphous state, a reflectance III of the portion in the crystalline state, and a reflectance IV of the portion in the amorphous state change. The absorption I of the portion in the crystalline state is always higher than the absorption II of the portion in the amorphous state, while the reflectance of the portion in the crystallin state is always higher than the reflectance IV of the portion in the amorphous state. That is, as is apparent from FIG. 5, even if the thickness of the first protection film 2 changes, the conventional optical information recording medium has a transmittance of 15% or more, so that the absorption is decreased to decrease the recording sensitivity.
In addition, an optical information recording medium using a diamond-shaped thin film as a reflective film is well known (Japanese Patent Laid-Open No. 4-252442). In this optical information recording medium, a difference between the absorptions of crystalline and amorphous portions is decreased, while a high thermal conductivity, a refractive index higher than that of a polycarbonate substrate, and a large difference between the reflectances of the crystalline and amorphous portions are kept. However, a transmitted light quantity tends to increase to decrease the recording sensitivity.
An optical information recording medium according to the third method in which information is reproduced using an optical phase difference, and the absorption is set with a certain degree of freedom is well known (Japanese Patent Laid-Open No. 5-145299). According to this method, however, a film thickness margin of each layer such as a recording film or protection film tends to be small.
Note that, as another conventional optical information recording medium, an optical information recording medium is well known in which the first protection film of a recording film has a film thickness of 100 nm, the absorptions of the recording film in the amorphous and crystalline states are 0.5 or more, respectively, and the film thicknesses of the second protection film and the recording film are set to have values which satisfy a predetermined inequality in association with the absorptions of the recording film in the amorphous and crystalline states (Japanese Patent Laid-Open No. 2-128330).
Taking light multiple reflection into consideration, an absorption contrast is set at 0.05 or less in the conventional optical information recording medium. However, the absorption of a medium corresponding to mark edge recording is not described in this prior application. In this prior application, only a decrease in difference between the absorption of a portion in the crystalline state and the absorption of a portion in the amorphous state is defined.